Internal combustion engines are increasingly equipped with injectors to which piezo actuators are assigned as adjustment drives. These types of injector have the advantage that very short valve opening times of the injectors can be achieved with them and thus a number of part injections are possible during one operating cycle of a cylinder of the internal combustion engine. In conjunction with a very high operating pressure −200 bar in the case of gasoline internal combustion engines for example—a very good preparation of the air-fuel mixtures is also possible in this way for direct injection of the fuel into the respective cylinders. In the case of a precise control of the injector this makes it possible to increase the efficiency of the internal combustion engine and especially to keep pollutant emissions low, which is necessary as a consequence of strict emissions legislation.
A method for controlling an internal combustion engine in the region of the lean limit is known from DE 197 06 126 C2. The method is used for internal combustion engines with leaner combustion with a fuel-air ratio λ, which is greater than in the stoichiometric case, i.e. with a lean mixture. This enables a greater efficiency of the internal combustion engine to be obtained. As the lean shift increases however the fluctuations between the combustion cycles rise, until finally inflammation dropouts occur. Uneven running values are determined for individual cylinders for fluctuations in the angular speed of the crankshaft. These are compared to predetermined uneven running values and fed to a regulator by means of which a maximum air-fuel ratio is adapted and the injection valves are activated accordingly.
A method for detecting combustion dropouts in a multi-cylinder internal combustion engine by evaluating the crankshaft speed is known from DE 195 44 720 C1. Segment time durations are measured, which the crankshaft needs during the operating cycles of the individual cylinders to pass through predetermined angular ranges. Furthermore these segment times are corrected with a correction factor which contains the mechanical tolerances of the rev counter. Uneven running values are calculated from the corrected segment times. The uneven running values are compared to a threshold value and misfiring is registered if the threshold value is exceeded.